Lanthanide Contraction is the steady decrease in the size of atoms/ions of lanthanide elements from $La$ to $Lu$.

• It is caused by the poor shielding of the nuclear charge by $4f$ electrons.
• A major consequence is that elements following the lanthanides (in the 3rd transition series) have atomic radii almost identical to their congeners in the 2nd transition series.
• Example: Zirconium ($Zr$) and Hafnium ($Hf$) have nearly identical radii ($\approx 160\text{ pm}$), making them very similar in chemical properties.